Angiogenesis, the process that new blood vessels form from existing vessels, is a critical process in development and response to environment and disease. Alterations in vascular development and angiogenesis play important roles in many diverse disease including diabetes, coronary artery disease, stroke, and cancer. More complete understanding of the mechanisms leading to appropriately controlled angiogenesis may lead to new approaches for prevention and/or treatment of these common debilitating diseases. Capillary tubules form from endothelial cells by a process called cell-hollowing tubulogenesis, which involves formation and fusion of intracellular vesicles to form a large intracellular vacuole that eventually becomes the extracellular lumen of the tubule. CLIC proteins are a family of chloride channel proteins which largely reside in intracellular membranes. Chloride permeability has long been recognized to play a role in intracellular membrane traffic but the specific proteins responsible for individual chloride conductance of intracellular membranes is still largely unknown. Recently, a CLIC protein has been shown to be essential for a specific cell-hollowing tubulogenic process in C. elegans and the family member CLIC4 has been implicated in tubulogenesis in vitro of mammalian endothelial cells. We have recently generated and characterized mice in which the gene for CLIC4 has been disrupted. Preliminary data from these mice provide startling and exciting new evidence that CLIC4 is indeed involved in angiogenesis. Our hypothesis is that CLIC4 plays a critical role in angiogenesis, perhaps by providing the chloride conductance necessary for the orderly membrane traffic and membrane fusion steps involved in cell-hollowing tubulogenesis of endothelial cells. We propose to: 1) Study CLIC4 (-/-) mice for abnormalities in vascular development and response to angiogenic stimuli; 2) Identify sequences and motifs within CLIC4 that are necessary for its function in cell-hollowing tubulogenesis; and 3) Investigate transport properties of membranes derived from cells undergoing tubulogenesis for an activity attributable to CLIC4 and explore how that effects tubulogenesis. The proposal thus is a hypothesis-driven basis science project that uses novel approaches and reagents to address a recently-identified key step in angiogenesis about which little is currently known mechanistically. The results will have clear clinical relevance to the many diseases in which angiogenesis plays a part.